A method according to the species is known from EP 0 716 158 A1. In that patent, a method is described for producing engine blocks with coated cylinder bores in which the engine block is initially cast from a metal and then dirt residue is removed from the walls of the cast cylinder bores so that initially an annular cleaned fresh metal surface is prepared. Then a plasma-sprayed layer is deposited on this pretreated surface and this layer is then finished.
The cylinder bores are usually prepared by machining them with corundum blasting followed by grease removal. The goal is to obtain a grease-free surface with a roughness (R) value of approximately 25 to 65 μm. Roughness (R) is determined by calculating the average peak to valley height of the surface of the machined cylinder bore. The problem is that the cylinder bores must be in precise position following preparation since after casting, the position of the cast cylinder bore can differ significantly from the prescribed value. In the manufacture of engine blocks from hypoeutectic aluminum by die casting, there is the additional problem that as a result of the casting process, inhomogeneities can develop especially in the lower part of the cylinder bores. Bubbles or pores can form in the material, caused by a shrinking process during casting (so-called shrink holes). During surface treatment and roughening, these pores or bubbles are exposed and can be further enlarged. Blasting residue and solvents or lubricant residues can remain in the open bubbles which results in poor adhesion of the applied tribological layer. Since the coating takes place at high temperatures, the solvent that remains in the open bubbles expands so that depressions and chips further worsens the adhesion of the tribological layer to the wall.
A goal of the present invention therefore is to provide a method of the type described above which provides grease-free surfaces of a certain surface quality in a method that is as simple as possible, onto which surface the thermally sprayed layers can be applied in simple fashion and with good adhesion.
The solution includes dry-cutting the interior in a method step without lubricant using a tool having a defined and/or undefined surface profile.
The term “dry cutting” means that no lubrication is used or that at most minimum lubrication with a volume flow of less than 150 ml/h is used in which the chips or the surface are considered to be dry.
Therefore, provision is made according to the invention for machining the bores when they are dry, for example by drilling, brushing, knurling, circular milling, or combinations of one or several of these methods.
The tool can have a defined surface profile so that after machining, a surface with a defined structure (S) results. Subsequent degreasing or cleaning and roughening are eliminated. Following surface treatment, a layer can be applied immediately by thermal spraying.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.